The present invention relates to a far-infrared camera lens having a wide angle and a lens unit and an imaging apparatus using the same. A far-infrared ray is light in a wavelength range of 8 μm to 12 μm, which includes a wavelength range of far-infrared rays that human beings emit. The wavelength range of far-infrared rays is much longer than a wavelength range for optical communication. A far-infrared camera is a camera that can sense an infrared ray, which is emitted from a human being or an animal, and can image the human being or the animal at night. In order to make it safer to drive an automobile at night, it is desirable to quickly and accurately recognize a human being or an animal that is present ahead.
An automobile illuminates a front side with a head lamp so as to recognize an image with reflected light. This is front recognition using visible reflected light. However, a distant place or a side where light emitted from a lamp does not reach is not visible in case of the method using visible reflected light. This is complemented by a far-infrared camera.
The body temperature of a human being or an animal is about 310 K, and a peak wavelength of black-body radiation at 310 K is about 8 μm to 12 μm. Accordingly, existence of a human being can be recognized by catching a far-infrared ray, which is emitted from a human being or an animal, using a far-infrared camera. A distant place other than a radiation range of a lamp can also be viewed because the far-infrared ray is not reflected light of the lamp. If an apparatus in which a far-infrared camera and an image processing system are combined is provided in an automobile, a human being or an animal that is present far away can be recognized early. Then, safety in driving an automobile at night will be improved.
It is preferable that a far-infrared camera for night observation be provided in an automobile. However, in order to make it possible, there are various difficulties to be solved. One of the difficulties is that the far-infrared camera is very expensive. In addition, another difficulty is that the resolution is not still sufficient. Thus, there is a problem that an optical system is defective. Furthermore, there is no light receiving elements which are cheap and suitable.
Since a far-infrared ray has low energy, it is not possible to detect the far-infrared ray by using a normal photodiode which uses a substrate, which is formed of Si, GaAs, InP, or the like having a wide band gap. Since the far-infrared ray has low energy, the ray can be received when PN junction is made with a semiconductor having a narrow band gap. However, since far-infrared energy is about a room temperature, the far-infrared ray cannot be detected when a light receiving element is at a room temperature. It is difficult to use the light receiving element for a vehicle if the light receiving element is not extremely cooled.
Therefore, for example, a thermopile detector, an SOI (silicon on insulator) diode, or a bolometer having sensitivity in a range of 8 to 12 μm is used as an imaging device of a far-infrared camera. Those described above are not light receiving elements having PN junction but elements which convert heat into electricity and non-cooled-type imaging devices. Currently, an imaging device having the number of pixels of 160×120 or 320×240 is used.
Here, discussion will be focused on an optical system. There is one problem in a lens material used to condense far-infrared rays. Germanium (Ge) is a material allowing an infrared ray to satisfactorily pass therethrough. Since germanium is a material allowing an infrared ray to satisfactorily pass therethrough and has a high refractive index (about 4 in the case of a far-infrared ray), germanium is an excellent infrared material. The transmittance of a far-infrared ray having a wavelength of 10 μm with respect to Ge is about 40 to 45%. However, in the case where antireflection coating is properly performed, the transmittance is about 90 to 98%.
However, Ge is a rare mineral the output of which is low. Ge is a limited natural resource and is very expensive. In addition, since Ge is extremely hard, it is necessary to make the form of the lens by cutting a large Ge lump and then make a surface smooth by grinding. This is a work that is performed over a long period of time using precise equipment. Since Ge is hard, a tool is also special. In the case when a Ge lens is used, a price is increased. It is difficult that an expensive far-infrared camera is widespread.
Chalcogenide glass is also known as a material of an infrared lens. Chalcogenide glass is glass containing chalcogen, such as chlorine, bromine, and iodine, and germanium. Since there is little absorption of infrared rays in the chalcogenide glass, the chalcogenide glass may be used for the infrared lens. Since the chalcogenide glass can be liquefied by heating, the chalcogenide glass can be molded in accordance with the shape of a mold. However, since the chalcogenide glass also contains germanium as a principal component, a material cost increases.
There is ZnSe as a material not containing Ge. ZnSe can be polycrystallized by using a CVD method, and then a lens can be obtained by scraping the polycrystallized ZnSe. In the same manner as Ge, it takes a manufacturing cost for cutting and grinding.
In order that a far-infrared camera is widely mounted in an automobile, it is necessary to manufacture the far-infrared camera at low cost. Therefore, it is necessary to develop a sensor capable of efficiently sensing far-infrared rays in a range of 8 μm to 12 μm and to manufacture a lens optical system at low cost. As described above, a best material for a far-infrared ray is germanium. However, germanium is an expensive material. Accordingly, as long as Ge is used, an inexpensive far-infrared camera cannot be made. Although the chalcogenide glass is also a next candidate, it is not possible to reduce the cost because the chalcogenide glass also contains a large amount of germanium. The ZnSe is also a candidate for infrared rays, but the ZeSe is not suitable as a camera lens because absorption of far-infrared rays is large.
Next, ZnS (zinc sulfide) is considered as a candidate. This is an inexpensive material. The far-infrared ray transmittance of ZnS is lower than that of germanium and far-infrared ray absorption of ZnS is larger than that of germanium. The transmittance at a wavelength of 10 μm is about 70 to 75%. In the case where antireflection coating is properly performed, the transmittance is about 85 to 90%. A refractive index of ZnS is lower than that of germanium. For this reason, ZnS is inferior to germanium in terms of properties as a lens. Moreover, it is also difficult to work with the ZnS. Currently, it may be possible to polycrystallize ZnS with a CVD method, to cut the polycrystalline ZnS in a cylindrical convex shape or a cylindrical concave shape, and to grind the polycrystalline ZnS so as to finally make a surface thereof smooth. However, since the ZnS is also a hard material, it takes cost to cut and grind the ZnS. For these reasons, there has been no infrared optical system realized by using a ZnS lens.
However, there are some proposals of far-infrared lenses using ZnS lenses. Patent Document 1 proposes a method of manufacturing a ZnS lens using a sintering process. In this case, ZnS powder is molded by hot compression using a lens-shaped mold.
Patent Document 2 proposes a method of manufacturing a lens as a polycrystalline ZnS sintered compact by molding ZnS by hot compression in a temperature range of 900° C. to 1000° C. and under the pressure of 150 to 800 kg/cm2.    [Patent Document 1] WO2003/055826    [Patent Document 2] JP-A-11-295501
One of the useful applications of a far-infrared camera is a night vision system which helps an automobile driver to perceive a pedestrian. This is a night-time pedestrian detection system using a far-infrared camera. Since a human being or an animal has considerably high body temperature, the human being or the animal emits infrared rays in a wavelength range of 8 μm to 12 μm. Existence of a human being or an animal in the street can be detected at night by using a camera that senses an infrared ray in the above wavelength range. Since it is not detection of reflected light, it is possible to detect a human being or an animal present in a distant place or an inclined portion where light of a lamp does not reach. It is expected, in an automobile running at high speed, to detect the existence of a human being or an animal positioned at a corner of a field of view that cannot be sufficiently viewed by reflected light of a head lamp. Accordingly, it is preferable to have a wide angle. In addition, in order to distinguish between a human being and an object body, high resolution is requested. Moreover, in the case of a system for a vehicle, the system should be small since there is no sufficient space in the vehicle.
However, in the case of a lens which is small and has a wide angle, a back focus BF tends to be shortened. The back focus is a distance from a back surface of a last lens of a lens system to an image surface (image formed surface). In the case of an infrared sensor, a window and a sensor (light receiving surface) are separated from each other by several millimeters. Accordingly, if a back focus is shorter than a distance between the window and an image surface, the last lens comes in contact with the window. For example, the distance between the window and the sensor is about 4 mm to 5 mm. The window has a thickness of 0.5 mm to 1.0 mm, for example.
Furthermore, it is necessary to attach a shutter outside the window. The shutter is a thin metal plate that is instantaneously opened and closed and has a limited thickness. The shutter should not be in contact with the last lens or the window. Therefore, if the sufficient back focus BF is not secured, there is no sufficient room for combination with a sensor. A desirable range of the back focus is determined from the size of a sensor. In the case where the distance between the window and the sensor surface is 4 mm to 5 mm and the thickness of the window is 0.5 mm to 1.0 mm as described above, a back focus of 6 mm or more is requested.